(CNN) - Former presidential candidate Rick Santorum stood by his tepid endorsement of former rival Mitt Romney on Tuesday, saying he supported the presumptive GOP nominee but would still try to hold his "feet to the fire."

"Well, I trust him more than Barack Obama," Santorum said on CNN's "John King, USA," when asked if he trusted Romney.

The former Pennsylvania senator last week announced he was forming Patriot Voices, a non-profit group that he says aims to help elect conservative politicians, including Romney, and make sure they're held accountable for their campaign promises.

Santorum said he supports "most of the tenants of" Romney's platform, but added his biggest concern was the health care bill Romney signed into law as Massachusetts governor–a plan that Santorum heavily attacked during the Republican primary earlier this year.

Santorum added, however, he believes Romney would work to repeal the federal health care law if he makes it to the White House.

Asked by CNN Chief National Correspondent John King who Romney should choose as a running mate, Santorum said he wasn't the best person to offer advice.

"The last thing I'm going to do is advise the person who vanquished me, if you will," Santorum said. "He did a pretty good job in winning the race and making decisions. I'm not going to be out there as a former foe making recommendations as to who he should pick."

And on the same day a new poll indicated Obama has a six percentage point edge over Romney in Pennsylvania, the former senator argued he thinks the Keystone State will be up for grabs by November.

"Barack Obama is not energy friendly. That's not going to play well in major parts of our state, and I think that's one of the reasons it will stay in play," he said.

CNN's electoral map puts Pennsylvania as a state that leans toward the president. Obama won Pennsylvania in 2008, 55%-44%, over Sen. John McCain.

Online Public Information File

Viewers with disabilities can get assistance accessing this station's FCC Public Inspection File by contacting the station with the information listed below. Questions or concerns relating to the accessibility of the FCC's online public file system should be directed to the FCC at 888-225-5322, 888-835-5322 (TTY), or fccinfo@fcc.gov.